1. Field of the Invention.
This invention pertains to the field of the control of automatic transmissions for motor vehicles. More particularly, the invention pertains to control of a hydraulic circuit used to actuate friction elements that produce various speed ratios of an automatic transmission.
2. Description of the Prior Art.
In a conventional automatic transmission, the vehicle operator changes the position of the gear selector lever among various operating ranges, including the park, reverse, neutral, drive, and low ranges. A series of mechanical linkages transmit movement of the gear selector to a manual valve located in a valve body of the automatic transmission. The manual valve includes a valve spool having several control lands, the spool located in a valve chamber and supported in the chamber for sliding movement along the axis of the chamber. The valve alternately opens and closes a connection between a fluid pressure source and a portion of a hydraulic circuit that is pressurized to produce the forward gear ratios, and another portion of the hydraulic circuit that is pressurized to produce reverse drive. The valve closes the fluid pressure source to the forward and reverse drive portions of the hydraulic circuit when the neutral range and park range are selected by the operator.
Use of a relatively large shift lever intrudes into the passenger compartment and has long been considered intrusive and incompatible with interior design and packaging. Furthermore, due to space limitation in the engine compartment, it is often difficult to locate the linkages and rods that connect the shift lever to the manual valve. This problem is made yet more difficult when the valve body is disposed at an awkward angle with respect to the motion-transmitting linkage.
Various attempts have been made to replace the gear range selector lever with push-button devices. Most recent attempts employ an electric motor, often a stepper motor, and a switching system for controlling the motor. A manual valve within the valve body of the transmission is moved within a valve chamber by the motor at positions within the chamber that corresponds to the selected range.